1. Field of the Invention
The present invention relates to a separator for a fuel cell and a fuel cell comprising the separator. More specifically, the present invention relates to a separator for a fuel cell that can be produced at low cost while possessing the same performance characteristics as a graphite separator produced by mechanical processing, and a fuel cell comprising the separator.
2. Description of the Related Art
Fuel cells are energy sources that produce electrical energy through electrochemical reactions between hydrogen or a hydrocarbon fuel and an oxidant, typified by oxygen. In recent years, fuel cells have received considerable attention as the most promising clean energy sources for the future.
Such a fuel cell includes a stack for generating electricity, a fuel supply unit for supplying fuel to the stack, and an oxidant supply unit for supplying an oxidant to the stack. The stack has a structure in which membrane electrode assemblies and separators are stacked alternately. In the membrane electrode assemblies, the fuel is oxidized and the oxidant is reduced to generate electricity.
Many types of fuel cells have been developed. Of these, polymer electrolyte fuel cells are currently investigated as electrochemical devices for producing electric power from hydrogen due to their relatively low operating temperature and the possibility of reducing the size of stacks. At present, it is estimated that there is a high possibility of putting polymer electrolyte fuel cells to practical use. Such a polymer electrolyte fuel cell includes a plurality of cells electrically connected to each other by separators or bipolar plates.
Separators are conductive plates to separate respective cells of a fuel cell stack. Each of the separators functions as a fuel electrode (i.e. an anode) in one of the adjacent two cells and as an air electrode (i.e. a cathode) in the other cell. The separator serves to block a fuel gas and air. Other roles of the separator are to ensure a flow path for the fuel gas and the air and to deliver an electric current to an external circuit. Thus, the separator is required to have high electrical and thermal conductivity, good corrosion resistance and low gas permeability.
Separators are usually produced using resin-impregnated graphite plates, carbon composite plates, metal plates, etc. Flow paths are formed in separators to assist in the flow of fluids. The separators play a role in dissipating heat generated from cells over the entire structure of a fuel cell stack. Excessive heat generated from the cells is collected through air- or water-cooling heat exchange. The collected heat can be utilized or wasted without use.
Fuel cell separators developed hitherto are classified into resin-impregnated graphite separators, carbon composite separators and metal separators by the kind of materials they employ.
A typical resin-impregnated graphite separator is produced by impregnating a graphite plate with a resin, followed by mechanical processing to form a gas flow path. Resin-impregnated graphite separators have been mainly used from the early stage of research and development of fuel cells owing to their very high electrical and thermal conductivity and good corrosion resistance.
A typical carbon composite separator is produced by molding a carbon/resin mixture. Carbon composite separators may have slightly lower electrical and thermal conductivity than resin-impregnated graphite separators. Thus, efforts to find suitable resins and optimize molding processes for the production of carbon composite separators have been made to fabricate fuel cells having similar performance characteristics to fuel cells using resin-impregnated graphite separators.
A typical metal separator is produced by processing/casting/molding a metal (typically stainless steel). Metal separators have very high electrical and thermal conductivity but they are disadvantageously susceptible to corrosion. Thus, suitable processes, such as surface treatment, coating and alloying, are used to improve the corrosion resistance of metal separators for use in fuel cell stacks.
Since a fuel cell separator is involved in the migration of electrons within a fuel cell, the conductivity of electrons penetrating the plate is the most important factor in the separator. A fuel cell separator should have a flow path to distribute gases into a gas diffusion layer. Other requirements of a fuel cell separator are good impermeability to reactive gases or ions and high chemical stability. Taking into consideration the characteristics of the market for automobiles and portable products, which are expected to be the main application fields of polymer electrolyte fuel cells, fuel cell separators should be light in weight and be able to be produced on an industrial scale.
Fuel cell separators are essential elements that account for the largest portion (about 30-60%) of the material costs of elements constituting a fuel cell stack. Accordingly, a reduction in the production cost of fuel cell separators would be most important for the commercialization of the fuel cell separators.
Graphite is the most commonly used material for separators. When it is intended to use graphite as a material for separators, the graphite undergoes mechanical processing to form flow paths for reactive gases present in the separators. That is, the formation of the flow paths in the separators requires additional processing, which makes it difficult to mass-produce the separators and entails a considerable production cost.
It is estimated that carbon composite separators have the potential to replace graphite separators owing to their low price and light weight. A typical carbon composite separator is produced by molding a carbon/resin mixture. A long operation time is needed for the molding and curling is likely to occur when one side of the separator is formed by the molding. Further, the separator is difficult to achieve sufficient electrical conductivity, which is the most important factor of the separator, due to the low electrical conductivity inherent to the polymeric material. Further, a high temperature is needed to melt the polymer during molding, such as compression or injection molding, which is simpler than mechanical processing. Accordingly, the type of the polymeric material used for the molding is limited to a thermoplastic resin. The molding process is highly energy consumptive, which is economically disadvantageous.
Coating is necessary to protect metal separators from corrosion. The coating requires a long operation time and incurs an increase in production cost. The use of coating materials results in a reduction in the electrical conductivity of the metal separators.